I am not 100% sure what you are asking for, so i just try to answer everything ;)
The futur I is not constructed with the present tense, but with werden + infitive:
"Ich werde es nehmen."
Your example "Ich nehmen es" is not correct because "nehmen" is either first/second person plural or infinitive. It could be:
"Ich nehme es" -present
"Wir/Sie nehmen es" -present
"Ich werde es nehmen" -futur I
BUT: German is not very consequent in using the gramatical futur for futur events. If it is clear from context that something lies in the future or if a temporal adverb in the sentence marks it as futur, german very often uses the gramatical present:
Ich nehme es morgen. I will take it tomorrow.
No, „Ich nehme es“ doesn't mean that.
The separable verb annehmen does however translate to to suppose / to assume, which I guess could just as well be expressed as I take it, …. But AFAIK that doesn't imply 100% certainty – at least not in German.
„Ich nehme an, dir hat das Geschenk nicht gefallen.“ / „Ich nehme an, dass dir das Geschenk nicht gefallen hat.“ – “I take it that you didn't like the present.”